Access your Jet.ne.jp (JETINTERNET) Account from an Email Program using IMAP

Jet.ne.jp supports IMAP / SMTP

This means you don't have to use Jet.ne.jp's webmail interface! You can
check your email and send messages using other email
programs (like Mailspring, Outlook Express, Apple Mail, or Mozilla Thunderbird).
Using desktop email applications can improve your workflow. They typically
offer more options, and your email is still available on your
computer when you're disconnected from the internet.